Japanese internet and telecommunications giant SoftBank Group Corp. is teaming up with a Saudi sovereign-wealth fund to create a multibillion-dollar technology-investment fund, in a partnership that combines deep pockets with one of the world’s most ambitious tech investors.

SoftBank, led by chief executive Masayoshi Son, is known for its bold and wide-ranging bets, ranging from Chinese e-commerce giant Alibaba Group Holding Ltd. and U.S. mobile carrier Sprint Corp. to U.K. chip designer ARM Holdings PLC, which it bought last...